Microwave heating utensil

ABSTRACT

A microwave heating utensil is provided with a microwave absorbent susceptor material which includes zinc and manganese. The susceptor material may be disposed in a layer on the surface of the body and a continuous cover layer overlying the susceptor material may be fused with the utensil body. A preformed layer comprising a susceptor-forming material including zinc and manganese, together with a glass moderator material, and a further preformed layer including the cover material in particulate form may be applied to the body by transferring these layers from a decal substrate. The continuous cover layer may be formed by fusion of the particulate cover material in heating, typically in an oxidizing atmosphere.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation-in-part of U.S. patent applicationSer. No. 826,087, filed Feb. 4, 1986.

BACKGROUND OF THE INVENTION

The present invention pertains to microwave heating utensils, processesof making such utensils and decals useful in the manufacture of suchutensils.

In typical microwave heating processes such as microwave cooking,microwave radiation is absorbed and converted to heat by the article tobe heated. Thus, in ordinary microwave cooking microwave energy isabsorbed by the food itself so that heat is generated within the food.In certain microwave heating processes, however, it is also desirable toapply externally generated heat as well. For example, meat cooked solelyby internal absorption of microwave energy does not develop thedesirable seared or browned exterior formed in conventional cookingprocesses. In cooking meat by microwave radiation, it is accordinglydesirable to apply externally generated heat to the surface of the meatso as to sear or brown the meat.

Utensils incorporating microwae absorbrent materials have been made andutilized in the past for such applications. The microwave-absorbentmaterial absorbs microwave energy and converts it into heat, thusheating the utensil. Accordingly, food placed in such a utensil andexposed to microwave energy is heated both by absorption of microwaveradiation within the food and by heat transfer from the hot utensil tothe food. Such utensils permit cooking and browning of meat in a typicalmicrowave oven.

One microwave heating utensil commercially available heretofore hasincluded a glass-ceramic body substantially transparent to microwaveradiation and a continuous microwave absorbent tin-laden layer on asurface of the body. The tin-laden layer typically is formed bycontacting the glass-ceramic with stannous chloride vapors. despite theextensive efforts devoted to development of microwave heating utensilsand utensil-making processes heretofore, there have still been needs forfurther improvement. there have been particular needs for moreeconomical utensil-making processes and for utensils which can befabricated by such processes.

SUMMARY OF THE INVENTION

One aspect of the present invention provides a microwave heating utensilwith an effective microwave-absorbent or susceptor material. The utensilpreferably includes a substantially microwave-transparent body and aparticulate microwave-absorbent or "lossy" susceptor material fixed tothe body. Preferably, the susceptor material is electrically conductive.The susceptor material may include at least one metallic oxide and atleast one metal in the unoxidized or reduced state. According to oneaspect of the invention, the metallic oxide component of the susceptormaterial may be magnetically responsive, and may include one or moreintermetallic oxides. As used in this disclosure, the term"intermetallic oxide" means a compound consisting of two or moredifferent metals and oxygen. One useful susceptor material includes ironoxides, nickel oxides and intermetallic oxides of iron and nickel suchas nickel-iron ferrite (NiFe₂ O₄) and also includes nickel in thereduced state.

The susceptormaterial may also include reduced or free metallic zinc.According to a further, particularly preferred, aspect of the invention,the susceptormaterial may include products formed by heating a mixtureof manganese and zinc in a firing operation as discussed below. Theseproducts may include some oxides of manganese and zinc. However, asubstantial portion of the zinc, and preferably substantially all of thezinc, remains in the reduced state. According to this aspect of thepresent invention, it has been found that oxidation of zinc duringformation of a zinc-based susceptor material is undesirable, and tendsto reduce themicrowave response of the susceptor material. Thus, thereduced or free metallic zinc is principaly responsible for themicrowave response of the susceptor material. As further discussedhereinbelow, the manganese serves to limit oxidation of the zinc andhence acts as an antioxidant. According to this aspect of the presentinvention, othermaterials which limit oxidation of zinc may also beemployed. The term "zinc antioxidant metal" as used in this disclosuremeans a metal which, when used in admixture with zinc, will tend tolimit oxidation of the zinc.

Preferably, the susceptor material is disposed in a thin susceptor layeron a surface of the body. According to a further aspect of theinvention, the utensil includes a continuous cover layer overlying thesusceptor layer so that the susceptor layer is disposed between thecover layer and the body. The cover layer is fused to the body, andprotects the susceptor layer from damage such as abrasion and detergentattack. Preferably, the body and the cover layer are formed fromelectrically nonconductive materials which may be glass or ceramicmateials. In the present disclosure, the term "ceramic" is used in abroad sense as encompassing glass-ceramics as well as conventionalceramics. The prferred materials for the utensil body are theglass-ceramics, notably those sold under the registered trademarkPYROCERAM. The preferred cover materials include glass.

The cover layer preferably is thin, and has a coefficient of thermalexpansion close to that of the body. Preferably, the cover layer isfused with the body only at spaced locations on the body surface, and atleast some of the interstices between particles of susceptor material inthe susceptor layer are unfilled. Although the present invention is notlimited by any theory of operation, it is believed that all of thesefeatures serve to minimize shear stresses caused by differences inthermal expansion of the body and layers, and hence prevent crazing andseparation of the cover and susceptor layes during the repeatedtemperature changes incident ot use of the utensil. Further, it isbelieved that the unfilled interstices contibute to the microwaveabsorptivity of the susceptor layer.

A further aspect of the present invention provides a simple, efficientand safe process for making a microwave heating utensil. In a processaccording to this aspect of the invention, a susceptor layer of aparticulate, susceptor-forming material is applied to a surface of theutensil body. The term "susceptor material", as used herein, refers to amaterial which is microwave absorbent. The term "susceptor-formingmaterial" as used in the present disclosure includes both susceptormaterials and materials which become microwave absorbent after furthertreatment.

Preferably, the susceptor-forming material as applied to the bodyincludes at least one metal in the reduced state. The susceptor-formingmaterial is heated. This heating step typically is part of a heating or"firing" operation used to fix the susceptor layer on the body.Desirably, this step is conducted in standard industrial equipment ofthe type normally used to fix or "fire" decorations and the like orglass orceramic ware and is conducted in an oxidizing atmosphere such asthe ambient air atmosphere commonly used in industrial firing equipment.Susceptor-forming materials such as ironnickel alloys which formmagnetically responsive oxides can be partially oxidized to form thesusceptormaterial of the finished article in situ, in the susceptorlayer on the utensil body during the heating step. Thesesusceptor-forming materials preferably are only partially oxidizedduring the heating step so as to provide in the finished article asusceptor material including both a metallic oxide and a metal in thereduced state. Particularly preferred susceptor forming materialsinclude mixtures of zinc and one or more zinc antioxidant metals such asmanganese in particulate form. Desirably, the zinc in thesesusceptor-forming materials is protected from oxidation bythezincantioxidant metal during the heating step. The susceptor layer asapplied to the body may also include an electrically non-conductivemoderator material such as glass, also in particulate form, in additionto the susceptor-forming material.

Preferably, one or more cover layers including a fusible, particulatecover material are applied so that the cover layer or layers overlie thesusceptor layer. During the heating step, the cover material fuses toform the continuous cover layer of the finished article and fuses withthe body so as to fix the cover layer, and hence the underlyingsusceptor layer, to the body, thereby joining the various layers to forma unitary utensil. Although the cover material flows to some extentduring fusion, the cover material preferably does not penetrate throughall of the interstices between the particles of susceptor material. Thecover material accordingly does not fuse to the body over the entiresurface of the body, but rather only at spaced locations on the surface,such as at the margins of the susceptor layer, at holes or openingsprovided in the susceptor layer, and at some of the interstices betweenparticles in the susceptor layer.

It is believed that the cover material limits entry of oxygento thesusceptor layer from the atmosphere and consequently limits oxidation ofthe susceptor-forming material during the heating step. When theparticles of cover material fuse to form a continuous coating, entry ofoxygen to the susceptor layer substantially ceases.

One or more of the cover layers may be applied in a pattern includingindicia, such as decorations or directions for use of the utensil. Thus,the same heating step as employed to fuse the cover layer also serves tofix the indicia on the utensil body.

The susceptor layer may be provided as a preformed layer on a decalsubstrate, and may be applied to the utensil body by transferring itfrom the decal substrate to the body. Preferably, the cover layer orlayers are also provided as preformed layers on the same decalsubstrate, so that all of these layers may be applied to the body in asingle transferring operation. The utensil making process according toparticularly preferred embodiments of the present invention thus greatlysimplifies production of the microwave heating utensil and eliminatesall of the difficult steps and hazardous materials associated with priorprocesses.

The present invention also provides a decal for making a microwaveheating utensil. The decal includes a susceptor layer attached to adecal substrate. The susceptor layer includes a particulate susceptorforming material and preferably includes a glass moderator material. Thesusceptor layer of the decal may also include a binder to maintaincohesion of this layer during application to the utensil body.Preferably, the decal also includes one or mroe cover layers comprisinga particulate, fusible cover material. The cover layers typically alsoinclude binders, and may further include pigments.

Other objects, features and advantages of the present invention will bemore readily apparent from the detailed description of the preferredembodiments set forth below, taken in conjunction with the accompanyingdrawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a fragmentary, schematic sectional view of a decal inaccordance with one embodiment of the present invention.

FIG. 2 is a view similar to FIG. 1 depicting a portion of a body,susceptor layer and cover layer in an intermediate stage of a processaccording to the present invention, using the decal of FIG. 1.

FIG. 3 is a fragmentary bottom view of a finished utensil made using thedecal and process of FIGS. 1 and 2.

FIG. 4 is a fragmentary, schematic, sectional view on an enlarged scale,taken along line 4--4 in FIG. 3.

FIGS. 5 and 6 are views similar to FIG. 1 illustrating decals accordingto further embodiments of the present invention.

FIG. 7 is a fragmentary, schematic, sectional view depicting a portionof a utensil made using the decal of FIG. 6.

FIG. 8 is a fragmentary bottom view of a utensil according to a furtherembodiment of the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

A decal according to one embodiment of the present invention, asillustrated in FIG. 1., includes a conventional paper decal substrate 10and a conventional, water soluble dextrin release layer 12 on thesubstrate. A susceptor layer 14 is disposed atop the release layer. Thesusceptor layer includes a coating of a varnish binder 15 and a singlelayer of fine particles 16 of an ironnickel alloy susceptor-formingmaterial adhering to the varnish base coating. Very fine particles 17 ofa glass moderator material are interspersed with the susceptorformingalloy particles. The susceptor layer in the decal is formed in a patterncorresponding to the desired pattern of the susceptor layer in thefinished article. As illustrated in FIGS. 1 and 3, the susceptor layerincludes a unitary covered area 18 with holes 20 extending through it atregular intervals.

The susceptor layer is formed by printing the varnish on the releaselayer in a pattern corresponding to the desired pattern of the susceptorlayer, applying a blend of the susceptor-forming alloy particles andglass moderator particles to the varnish and then drying the varnish sothat the varnish holds the particles in place. After drying, looseparticles are removed as by brushing. After this operation, only thosealloy particles which are in direct contact with the varnish coatingremain. Thus, if several layers of particles are deposited one atop theother on the varnish coating during the application step, only typelowest layer remains after the removal step. Substantially all of theparticles are removed from those areas devoid of varnish, thus formingthe holes 20. A process of forming a particulate layer in a desiredpattern by adhesion of the particles to a tacky varnish layer is wellknown in the decal making art, and is commonly referred to as a "litho"process.

A first cover layer 22 comprising fine particles of a low-melting,low-expansion glass in an acrylic binder is disposed atop susceptorlayer 14, portions of the first cover layer extending through the holes20 in the susceptor layer. The first cover layer also includes a minoramount of a conventional pigment, such as oxides of iron, chromium andcobalt. The first cover layer 22 includes a continuous portion overlyingthe entire susceptor layer and extending beyond the margins 26 of thesusceptor layer. First cover layer 22 also includes isolated portions 27remote from the continuous portion, the isolated portions being in theshape of lettering or other indicia, as seen in the finished utensil(FIG. 3). A second cover layer 24 of composition similar to that of thefirst cover layer, but without the pigment, is disposed atop the firstcover layer.

The cover layers typically are formed by screen printing a mixture ofthe glass particles in a vehicle comprising the binder and an organicsolvent for the binder. The technique of applying a glass and binderlayer by screen printing is well known in the art of making decals fordecoration of ceramic ware and glassware.

The particle sizes and the thicknesses of the various layers are greatlyexaggerated in the drawings for clarity of illustration. Typically, thealloy particles are less than about 40 microns and preferablyless thanabout 27 microns in diameter, whereas the glass particles in the coverlayers are preferably about 2 to 4 microns in diameter. Each of thecover layers in the decal typically is less than about 10 microns thick.

In one process according to the present invention, the decal illustratedin FIG. 1 is used to form cover and susceptor layers on a surface 29(FIG. 2) of a glass-ceramic body 28 in the form of a conventionalskillet. The decal is moistened so as to soften and dissolve releaselayer 12 and juxtaposed with the body so tha the substrate faces towardsthe body. The substrate is then removed, leaving susceptor layer 14 andcover layers 22 and 24 on surface 29. These steps are the same asemployed in applying a conventional decal of the type known in the ecalart as a "water" or "slide-off" decal.

After the susceptor and cover layers have been applied, the body and theocver and susceptor layers are heated under an oxidizing atmosphere suchas air in a conventional oven of the type commonly utilized to fixdecorative decals on ceramic ware, and are gradually cooled to roomtemperature. In the heating step, the varnish and acrylic bindersoxidize to form gaseous oxidation products which dissipate rapidly. Theglass particles in the cover layers soften and melt, and hence flow toform a single, continuous imperforate cover layer 30 (FIG. 4), the coverlayer being fused to the body at the margins 26 of the susceptor layerand at each of the holes 20 in the susceptor layer and at each of theholes 20 in the susceptor laye, as schematically depicted in FIG. 4. Theintegral cover layer in the finished article typically has a totalthickness about one half the total thickness of the two cover layers inthe decal. This decrease in thickness is believed to result from loss ofthe binders and flow of the glass to occupy the space occupied by thebinders prior to heating.

The alloy particles in the susceptor layer oxidize to yield the finalsusceptor material. Preferably, the metals in the susceptor layer arenot completely oxidized during the heating step. Although the presentinvention is not limited by any theory of operation, it is believed thatthe cover layer serves to limit oxidation of the susceptor layer. Body28 is substantially nonporous, so that oxygen from the surroundingatmosphere cannot enter the susceptor layer through the body. Prior tofusion of the glass particles, the cover layers are porous and permitsome entry of oxygen to the susceptor layer. It is believed that, uponfusion of the glass particles, the resulting unitary cover layer 30becomes substantially nonporous and substantially stops furthertransmission of oxygen to the susceptor layer. The iron in the alloytypically oxidizes to a substantially greater extend than the nickel.Thus, at least a portion of the nickel remains in the reduced orunoxidized state in the final susceptor material, after heating. Thefinal susceptor material also includes oxides of iron and nickel. It isbelieved that the oxides present include intermetallic oxides of ironand nickel, i.e., compounds including iron, nickel and oxygen. It isbelieved that the intermetallic oxides formed include some nickel-ironferrite or NiFe₂ O₄.

After theheating step, the susceptor material remains in particulateform, although there may be some rearrangement of the material intodifferent particles than originally present in the susceptor layerbefore heating. Thus, some of the reduced or unoxidized nickel isbelieved to segregate during the heating step from the oxide particlesformed, to yield microscopic particles of substantially pure reduced orunoxidized nickel, which may take the form of microscopic fibers ordroplets. Also, there may be some sintering or fusion of adjacentparticles in the susceptor layer during the heating operation.

After heating, the interstices 32 between the particles 16 of the finalsusceptor layer 14 are not completely filled by glass. The glass of thecover layer may flow through some of the larger interstices 34 to joinand fuse with the body at such interstices. Further, the glass moderatorparticles incorporated in the susceptor layer fuse during heating todeposit additional glass in the interstices between particles ofsusceptor material. Nonetheless, at least some of the intersticesbetween particles of susceptor material are left unfilled. The finishedarticle thus incorporates susceptor material in the form of an unpackedlayer of particles 16 with some unfilled void spaces or interstices andalso includes a continuous vitreous cover layer 30 closely overlying thesusceptor layer, the cover layer being fused to the body only at spacedlocations on the body surface, viz, at the margins 26 of the susceptorlayer, at holes 20 and at certain of the larger interstices 34. As mostclearly seen in FIG. 4, portions of the fused cover layer 30 betweenthese spaced locations bridge over the susceptor layer but are not fusedto the body. Accordingly, although the susceptor layer in the finishedutensil is completely enclosed by the cover layer and the body, theparticles 16 in the susceptor layer are not tightly bound in a glassymatrix.

Although the present invention is not limited by any theory ofoperation, it is believed that the controlled flow of the glass covermaterial to form a continuous layer without completely filling theinterstices in the susceptor layer is related both to the relativelyhigh viscosity of the glass at the temperatures used for the heatingstep and also to the effects of surface tension. Typically, the coverlayer is heated to a temperature just slightly higher than the meltingpoint of the cover material, i.e., about 690° C. to about 710° C., andmaintained at that temperature for about 4 to 7 minutes before cooling.It is believed that the molten cover material does not wet the particlesof the susceptor layer, so that the cover material is excluded at leastfrom the finer interstices by surface tension. Contraction of thesusceptor material during cooling may also contribute to formation ofunfilled interstices in the susceptor layer. The coefficient of thermalexpansion of the susceptor material typically is greater than thecoefficient of expansion of the body and of the cover material. When theutensil is cooled the particles of susceptor material contract to agreater extent than the body and the cover layer. Such contraction ofthe particles tends to open the interstices between the particles.

In the finished utensil, the cover layer effectively retains thesusceptor material on the body surface, and protects the susceptor layerduring use of the utensil. Thus, the cover layer prevents abrasion ofthe susceptor layer. Moreover, the cover layer protects the susceptormaterial from further oxidation during use of the utensil, as uponheating incident to microwave exposure. The cover layer also protectsthe susceptor material from chemical attack, including detergent attackduring cleaning, and prevents leaching of the susceptor material fromthe susceptor layer. Thus the cover layer greatly prolongs the usefullife of the susceptor layer and also prevents contamination of thesurroundings by the susceptor material.

The finished utensil may be employed in microwave cooking. A piece ofmeat or other food may be placed on the interior surface of the utensilbody (the surface opposite from surface 29) and theutensil may be placedin a standard microwave oven. The utensil body may be supported on ashelf of the microwave oven by the feet 31 projecting from the utensilbody so as to maintain surface 29, and hence the cover and susceptorlayers, remote from the shelf. Upon exposure of the utensil to microwaveradiation, susceptor layer 14 absorbs microwave energy and converts thesame to heat thereby heating the adjacent wall portions of the utensiland hence the food in contact therewith.

Although the present invention is not limited by any theory oroperation, it is believed that absorption of microwave energy andconversion of the same to heat by the susceptor layer involves bothinteraction of the oxide particles with the magnetic component ofthemicrowave energy and induction of eddy currents in the susceptorlayer. It is believed that the unfilled interstices adjacent theparticles of susceptor material in the susceptor layer enhance theresponse of the particles to magnetic excitation. It is also believedthat the reduced or unoxidized metal present in the susceptor layerenhances the electrical conductivity of such layer and facilitatesgeneration of eddy currents in the susceptor layer. Moreover, it isbelieved tha the cover material does not substantially insulate theparticles of susceptor material from one another, and hence does notsubstantially impede the flow of eddy currents within the susceptorlayer. These factors are believed to contribute materially to theeffective conversion of microwave energy to heat which occurs when theutensil is exposed to microwave energy. Moreover, the electricallynonconductive cover layer closely overlying the susceptor layer inhibitsformation of electrical arcs at the surface of the susceptor layer. Thecover layer thus greatly facilitates use of susceptor materials such asparticulate metallic oxides and metals which would otherwise tend togenerate arcs during microwave exposure.

Preferably, the susceptor layer generates enough heat to raise thetemperature of the adjacent portions of the utensil body to about 205°C. or more, hot enough to sear or brown meat. As will be readilyappreciated,the utensil is subjected to repetitive thermal cyclingduring use. The materials included in the susceptor layer typically havecoefficients of thermal expansion markedly higher than the coefficientof thermal expansion of the body material. For example, glass-ceramicsof the types sold under the registered trademark PYROCERAM havecoefficients of thermal expansion on the order of about 10⁻⁶ cm/cm/°C.whereas metallic nickel and nickel-iron ferrite have coefficients ofexpansion on the order of about 10⁻⁵ cm/cm/°C. Such a great disparitybetween coefficients of expansionin adjacent layers of the utensil wouldtend to cause significant differential expansion and hence would tend toinduce substantial stresses at the interface between the layers.

It is believed, however, that these stresses are minimized by theunfilled interstices in the susceptor layer. These interstices arebelieved to provide room for expansion of the susceptor materialparticles. The temperatures attained during the use of the utensiltypically are lower than the temperatures attained during the heatingstep used to make the utensil. Accordingly, the expansion of thesusceptor particles during use typically is less than the contraction ofthose particles during the cooling step in manufacture. It is,accordingly, believed that such expansion does not completely close allof the unfilled interstices in the susceptor layer, and hence does notinduce appreciable compressive stress in the susceptor layer. Moreover,it is believed that attachment of the cover layer to the body only atspaced locations on the body surface further minimizes stresses causedby differential expansion of the susceptor material and body. Thus, thecover layer may flex or bulge slightly away from the body when thesusceptor material expands upon heating. These features of the utensileffectively compensate for differential thermal expansion of thesusceptor material and the body. Thus, the susceptor material may beselected substantially without regard for its thermal expansioncoefficient.

In the processes described above, an iron-nickel alloy susceptor-formingmaterial is employed to form a susceptor material comprising oxides ofiron and nickel, and nickel in the reduced state. Iron-nickel alloyswith iron contents ranging from about 20% to about 80%, the remainderbeing nickel, and particularly alloys containing about 70% iron andabout 30% nickel can be used in this way. Iron and nickel may beprovided in forms other than alloy particles. Thus, thesusceptor-forming material may be a mixture including fine particles ofiron and fine particles of nickel. Such mixtures may be formed byreduction of mixed, gaseous, carbonyls of the iron and nickel. Althoughuseful results may be obtained with such mixtures, susceptor materialsfomed by oxidation of iron and nickel alloy particles typically providemore effective and more consistent heat generation than susceptormaterials formed by oxidizing mixture of iron particles and nickelparticles.

Pure zinc may also be used as a susceptorforming material. A mixture ofzinc oxide and zinc in the reduced state is formed upon partialoxidation of a pure zinc susceptor-forming material.

It has been found, however, that susceptorforming materials containingmixtures of one or more zinc antioxidant metals such as manganese,together with zinc provide finished utensils with particularly desirableproperties. Mixtures containing separate particles of manganese and zincat zinc:manganese ratios of between about 80:8 and about 80:18 by weightare preferred. Zinc:manganese ratios of about 80:13 are particularlypreferred. Although the present invention is not limited by any theoryof operation, it is believed the superior performance of the zinc andmanganese mixture vis-a-vis pure zinc occurs because the zinc in themixture is less susceptible to oxidation than pure zinc. Thus, pure zincsusceptor-forming materials tend to oxidize to excess during the heatingstep in the manufacturing process, whereas the zinc and manganesesusceptor-forming mixtures typically yield good susceptor layers in thefinished utensils. Also, the zinc and manganese susceptor-formingmixtures provide properties in the finished utensil superior to thoseachieved with iron or iron-nickel alloys. Thus, finished susceptorlayers made with the preferred zinc/manganese mixture susceptor-formingmaterials aremore resistant to arcing and undesired localized heating inservice than are comparable layers made with iron-nickel alloys.

The thermal expansion coefficient of the cover layer typically ismatchedas closely as is practicable with that of the underlying body. Thus, foruse with a glass-ceramic body, the glass utilized in the cover layerpreferably has a coefficient of expansion of about 60×10⁻⁷ cm/cm/°C. orless. The glass utilized in the cover layer preferably has a meltingpoint below about 710° C. and good resistance to ion replacement by themetals of the susceptor-forming material at the temperatures attainedduring the heating step. The cover material preferably also has goodabrasion resistance and good resistance to chemical attack, particularlyattack by detergents under the conditions encountered in use of thefinished utensil. Glasses having the desired combination of propertiesare well known in the art. However, the low melting, low expansion glasscommonly referred to as "fluxes" are preferred.

Further to minimize stress at the points of fusion between the coverlayer and the body, the cover layer in the finished utensil should bethin. Preferably, the composite unitary cover layer resulting fromfusion of glass particles in the two cover layers of the decal isbetween about 10 microns and about 6 microns thick, the range betweenabout 8 microns and about 6 microns being particularly preferred. Asused in this disclosure with reference to a layer,the term "thickness"means the average thickness of the layer excluding any visible holes oropenings in thelayer. The thickness of the cover layer in the finishedutensil depends upon the proportions of cover material and binder in thecover layers of the decal, and the thickness of these layers. Theseparameters may be adjusted so that the total mass of cover material perunit area in the cover layers of the decal corresponds to the mass perunit area of a fused cover layer having the desired thickness. To form acover layer between about 6 microns and about 10 microns thick, about2.64 mg/cm² to about 4.4 mg/cm² of glass having a density of about 4.4gm/cm³ are required. Stated another way, the total volume per unit areaof the glass particles in the cover layeres of the decal corresponds tothe thickness or volume per unit area of the desired fused cover layerin the finished utensi, viz., about 6×10⁻⁴ cm³ /cm² to about 10×10⁻⁴ cm³/cm².

The moderator included in the susceptor layer reduces the electricalconductivity of the susceptor layer, and hence moderates the temperatureattained by the utensil during microwave exposure in use. The ratio ofmoderator to susceptor-forming material in the decal, and hence theratio of moderator to susceptor material in the finished utensil, isselected to provide the desired temperature during use of the utensil.The moderator may be omitted entirely to provide higher temperaturesduring use. The glass utilized as the moderator preferably has a meltingpoint and coefficient of expansion similar to those of the covermaterial. The moderator is protected from detergent attack during use ofthe utensil, but is intimately exposed to the metals of the susceptormaterial. Accordingly, glass compositions which are particularlyresistant to ion replacement are preferred as moderator materials; theglass available under the designation "flux number 1803" from CorningGlass Works is particularly preferred as a moderator.

Moderators are particularly useful with zinc/based susceptor-formingmaterials. With the particularly preferred zinc/manganesesuseptor-forming materials, the moderator preferably constitutes about2% to about 12%, more preferably about 5% to about 9% and mostpreferably about 7% by weight of the susceptor layer, excluding bindersprior to the heating step. Thus, the ratio of susceptor-forming material(zinc and manganese together) to moderator be weight in the susceptorlayer may be about 88:12 to about 98:2, and more preferably about 91:9to 95:5. The most preferred susceptor layers include 80 partsparticulate zinc, 13 parts particulate manganese and 7 parts glassmoderator material.

A decal according to a further embodiment of the present invention,illustrated in FIG. 5, incorporates only one cover layer 122. The singlecover layer of the decal incorporates the entire mass of glass requiredto form a cover layer of the desired thickness in the finished article.Also, both cover layer 122 and susceptor layer 114 ar made by a silkscreen process. Cover layer 122 includes a mixture of glass particlesand acrylic binder similar to that employed in the cover layers of thedecal of FIG. 1. Susceptor layer 114 includes metal particles 116 andmoderator particles 117 in the same type of acrylic binder. Where thesusceptor layer is applied by a silk screen process, the metal particlesin the susceptor layer desirably should be less than about 20 microns indiameter, preferably less than 10 microns in diameter and mostpreferably less than about 5 microns in diameter. The moderatorparticles are desirably about the same size or smaller than the metalparticles. With metal particles about 5 microns in diameter, themoderator particles may also be less than about 5 microns in diameter,most typically about 1/2 to about 3 microns. The decal of FIG. 5 isarranged for so-called "heat release" application to the utensil body.Thus, the postion of the substate relative to the other layers is thereverse of that illustrated in FIG. 1. In the decal of FIG. 5, substrate110 is disposed adjacent the cover layer rather than the susceptorlayer. A waxy release layer 112 as commonly employed in heat releasedecorative decals is disposed between the substrate and cover layer. Alayer 140 of a heat activatable adhesive as commonly used in heatrelease decorative decals is disposed on the susceptor layer 114. Inuse, the decal is placed on the utensil body so that adhesive layer 140is in contact with the body surface and the decal is warmed so as tosoften the wax of release layer 112 and activate the adhesive of layer140. Substrate 110 is then peeled away from the cover layer, leaving theremainder of the decal behind on the utensil body. These steps are thesame as commonly utilized to apply conventional heat release decorativedecals to ceramic ware and glassware. The heating step after applicationof the cover and susceptor layers to the body is substantially the sameas employed in the process described above. The adhesive of layer 140oxidizes to gaseous oxidation products, which dissipate along with theoxidation products of the acrylic binders in the susceptor and coverlayers.

The decal illustrated in FIG. 6 is a heat release decal similar to thatshown inFIG. 5 and incorporates a similar substrate 210, cover layer222, susceptor layer 214 and adhesive layer 240. However, an underlayer242 including a mixture of glass particles and binder similar to thatemployed in the cover layer is interposed between susceptor layer 214and adhesive layer 240. When the decal is applied to the utensil body,underlayer 242 is interposed between the susceptor layer and theoriginal surface of the body. Upon heating, the glass of underlayer 242fuses with the body to provide a continuous vitreous underlayer on thesurface of the body 228 as depicted in FIG. 7, the underlayer forming anintegral part of the body. Cover layer 222 fuses with the underlayer,and hence with the body, at the spaced openings 220 in the susceptorlayer and at the margins of the susceptor layer. There may be some flowof glass from the cover layer and the underlayer into the intersticesbetween adjacent particles in the susceptor layer, and such flow mayserve to fuse the cover layer withthe underlayer, and hence with thebody, at some larger interstices 234 between particles in the susceptorlayer. However, such flow typically is insufficient to completely fillall of the interstices between the particles of susceptor material.Accordingly, as in the embodiments descibed above, the cover layer isfused to the body only at spaced apart locations, viz, the openings, themargins of the susceptor layer and thelarger interstices.

The glass of the underlayer preferably is a low melting glass similar tothat used for the cover layer. however, the glass of the underlayer maybe selected to provide a coefficient of thermal expansion intermediatebetween those of the underlying body material and the cover material,thereby to provide a gradation in coefficient of expansion between theunderlying body material and the cover material. The thickness of theunderlayer in the finished utensil may be about 1.5 to about 4 microns.

The utensil illustrated in FIG. 8 is similar to that illustrated inFIGS. 3 and 4. However, in the utensil of FIG. 8 the openings 320 in thesusceptor layer are in the form of narrow, elongated, linear slitsrather than circular holes. The linear openings are connected to oneanother at their ends so that the openings subdivide the susceptor layerinto a plurality of zones 350, each such zone being isolated from theadjacent zone by an intervening opening. As the individual zones 350 ofthe cover layer are isolated from one another by the fused cover layerand body at the openings 320, eddy currents generated upon exposure ofthe utensil to microwave energy do not flow between the zones, butinstead are confined to circulation within individual zones. As will bereadily appreciated, the attern of the susceptor layer in the finishedutensil can be altered as desired by varying the printing pattern of thesusceptor layer in the manufacture of the decal.

Application of the cover layer, susceptor layer, and underlayer, ifused, to the utensil body by means of a single decal is preferredbecause the decal permits application of these layers in the desiredpatterns and with the desired thickness or mass per unit area in asingle, economical operation. Moreover, the layers on a printed decalcan be inspected before application to the utensil body to preventapplication of imperfect layers and thus avoid loss of utensil bodies.However, some or all of these layers may be applied to the body withoutusing a decal. Thus, particulate susceptor-forming material or alloy maybe deposited directly on the utensil body to form the susceptor layerand particulate cover material may be deposited directly over theso-formed susceptor layer. The susceptor-forming material and covermaterial may be deposited by any suitable technique as, for example, bysilkscreen printing on the surface of the body, utilizing mixtures ofbinders and solvents similar to those employed in manufacture of thedecal by a silkscreen printing process. Also, a vitreous underlayer maybe provided on the surface of the body by subjecting the body to aglazing process as commonly employed in the ceramic industry beforeapplying the susceptor layer and cover layer. Alternatively, thesusceptor layer may be applied by means of a first decal and the coverlayer may be applied by means of a second decal or by direct applicationof the cover material over the susceptor layer.

The substrate, release coating, adhesive and binder employed in thedecal serve merely to maintain the susceptor forming material and/orcover material as coherent, performed layers and to facilitateapplication of these layers to the utensil body. Any conventionalcombination of substrate, release coating, adhesion and binder may beemployed, provided that these components do not adversely affect thefinished utensil or the processses occurring during the heating step.Thus, the binders, adhesives and release coating should oxidize readilyduring the heating step to form gaseous products rather than solidresidues which may contaminate the finished utensil. In this and otherrespects, the considerations involved in selection of a substrate,release coating, adhesive and binder system for a decal according to thepresent invention are the same as those which apply with regard to adecorative decal for glass or ceramic ware. Particularly preferred heatrelease systems are set forth in U.S. Pats. Nos. 4,068,033 and4,117,182, the disclosures of which are incorporated by referenceherein. Water release systems may also be utilized. Further, the decalmay be formed with a combustible substrate such as nitrocellulose,without any release system. In this arrangement,the susceptor and/orcover layers are applied to the body by placing the entire decal on thebody, and these layers are transferred from the substrate to the body byburning the substrate, as during the heating step.

Susceptor-forming materials other than those mentioned above may also beemployed. Thus, metals and combinations of metals other thaniron/nickel, zinc, and zinc/manganese may be employed. Zinc ismostpreferably used in conjunction with one or more zinc antioxidant metalssuch as manganese. Nonmetallic conductive materials can be utilized. Ina variant of the process, the heating step may be conducted in an inertor reducing atmosphere. However, typical industrial heating processes,such as thefiring step used in application of decorations to ceramicarticles are conducted in an air atmosphere. The standard equipment usedfor those firing processes is not arranged to maintain an inert orreducing atmosphere. It is thus greatly preferred to use an airatmosphere in the present process, os as to avoid the expanse of aninert or reducing atmosphere and the expense of spherical equipment formaintaining such an atmosphere.

The equilibrium temperature attained by the susceptor material, andhence the utensil body, during exposure to microwave radiation will varywith the composition of the susceptor material, the thickness of thesusceptor layer and the heta losses imposed on the utensil by the foodor other article to be heated. Where the susceptor material is magnetic,the magnetic response of the susceptor material, andhence the rate ofheat evolution within the susceptor layer declines markedly as thesusceptor material approaches its Curie temperature. As heat iscontinually lost from the susceptor material, the equilibriumtemperature attained by a magnetic susceptor material typically will besomewhat less than the Curie temperature.

The equilibrium temperature attained by the susceptor material increasesas the thickness or mass of susceptor material per unit area of thesusceptor layer increases. The mass per unit area of the susceptormaterial may be stated as "metal mass per unit area." As used herein,the term "metal mass per unit area" refers to the mass per unit area ofthe metal or metals in the susceptor layer, and hence excluedes the massof the oxygen incorporated in the susceptor layer. The area of thesusceptor layer, as referred to herein, excludes the area of any visibleholes or openings. For a susceptor material formed by in situ oxidationof a reduced metal susceptor forming material, the metal mass per unitarea in the susceptor layer of the utensil will be equal to the mass ofsuseptor-forming material per unit area initially applied to the utensilbody.

The desired equilibrium temperature varies with the intended use of theutensil. For microwave cooking, equilibrium temperatures between about260° C. and about 326° C. typically are desired. Susceptor layers formedby partial oxidation of an iron-nickel alloy susceptorforming materialand having a metal mass per unit area between about 6 mg/cm² and about18 mg/cm² and most preferably about 12 mg/cm² provide equilibriumtemperatures within this desired range and accordingly are preferred forcooking utensils. Susceptor layers formed by partial oxidation of a zincsusceptor-forming material, and containing a glass moderator in anamount equal to about 0.35 to about 0.5 times the metal mass of thesusceptor layer, provide equilibrium temperatures within the desiredrange at a metal mass per unit area of between about 5 mg/cm² and about15 mg/cm², most preferably about 10 mg/cm². With a susceptor layerformed from the preferred zinc and manganese mixture susceptor-formingmaterial, and containing the preferred amounts of glass moderator usedwith this material, the metal mass per unit area (the mass per unit areaof the zinc and manganese together) preferably is about 4 mg/cm² toabout 10 mg/cm², and more preferably about 6.3 to about 7.9 mgcm².

The present invention may be used with utensil bodies incorporatingmaterials other than the glassceramic materials referred to above. Forexample, utensil bodies formed from conventional ceramics or glass maybe employed in the present invention. Although glass is preferred as thecover material and underlayer material, other fusible, nonconductivematerials may be used. Thus, ceramic compositions may be employed ascover mateials, the ceramic being fired or partially fused during theheating step. Also, the cover material may include a thixotropic agentto limit its flow during the heating step. Suitable thixotropic agentsfor use with a glass cover material include fine particles of arelatively high melting glass which do not melt at the temperaturesemployed in the heating step. Preferably, the body material, the covermaterial and the underlayer material, if employed are substantiallytransparent to microwaves, so that microwave energy incident on theutensil will reach the susceptor layer.

The examples set forth below illustrate certain aspects ofthe presentinvention:

EXAMPLE 1

A dextrin coated substrate paper is printed with a 3 micron thick layerof a linseed oil varnish in a pattern comprising a continuous generallyoctagonal spot about 21 cm wide with holes about 1.5 mm indiameterextending through the varnish layer, the holes being arranged in arectilinear grid pattern as illustrated in FIG. 4 at 2 mm center tocenter spacing. A 70% iron-30% nickel alloy powder having a maximumparticle size of about 27 microns is applied to the tacky varnishcoating. Loose metal particles are removed byapplying flour to the metalcovered varnish layer and brushing off the flour. A vehicle is made bymixing 30 parts by weight methyl methacrylate, 10 parts by weightdodecal zenesulfonic acid and 60 parts by weight of an aromatichydrocarbon solvent. A first glass coating mixture is made by mixing onepart by weight of the vehicle with one part by weight of Corning No.1803 low expansion, low melting glass flux having a mean particle sizeof about 3 microns together with 0.01 parts by weight chromium oxideblack pigment having a similar particle size. A substantially continuouslayer of this first mixture is applied by screen printing through a 330mesh screen so that the continuous layer covers the entire areaencompassed by the alloy layer and extends about 2 mm beyond the alloylayer at the margins thereof. Additional portions of the first orpigmented mixture are deposited directly onto the dextrin layer in areasremote from the alloy layer, these further portions being in the shapeof letters. Application of the first coating is controlled so that,after the first coating is cured by drying in air to evaporate thesolvent, the resulting cover layer of glass in acrylic binder is about 6microns thick. A second glass coating mixture is prepared according tothe same formulation as used for the first glass coating mixture, savethat the pigment is omitted. This second mixture is applied over thecured first cover layer sing a 390 mesh screen and cured by evaporationof the solvent to form a second cover layer, also about 6 microns thick.The decal formed by these steps is substantially as illustrated in FIG.1.

The decal is moistened and juxtaposed with a glass-ceramic skillethaving a generally square bottom wall about 25 cm on a side. Thesubstrate of the decal is removed, leaving the susceptor or alloy layerand the cover layers of the decal on the outside or non-food contactingsurface of the skillet bottom wall. The skillet, with the susceptor andcover layers thereon, is passed through a conventional, conveyorizedoven wherein the skillet is heated in air to about 700° C. over a periodof about 40 minutes, held at about 700° C. for about 6 minutes and thencooled to room temperature over a period of about 10 minutes.

The resulting microwave heating skilled may be effectively heated tomeat browning temperature by exposure to microwave radiation in aconventional microwave oven as commonly utilized for home cookingpurposes. The skillet is substantially unaffected by repeated thermalcycling. The microwave absorbent susceptor layer is effectivelyprotected from abrasion and from detergent attach by the fused,continuous glass cover layer. The lettering resulting from theletter-shaped portions of the first or pigmented cover layer in thedecal provides permanent indication as to directions for use. Theseindicia are likewise resistant to damage during use.

EXAMPLE 2

A decal is fabricated using a mixture of 80 parts by weight metalliczinc dust and 13 parts by weight manganese dust each having anarithmetic mean particle size of less than about 5 microns. The zincdust and manganese dust are glended with 13 parts by weight particles ofCorning No. 1803 flux inparticles smaller than about 2 microns. Thesecomponents are mixed with an acrylic binder to provide a susceptor layercoating composition, including about 1 part by weight binder per part byweight zinc and manganese, taken together. The susceptor layer of thedecal is fabricated by screen printing this coating composition on thedextrin-coated release paper so as to provide about 7 mg/cm² of zinc andmanganese, taken together. In other respects,the decal fabricationprocess is the same as employed in Example 1. The decal is applied to autensil body and heated by the same processes as employed in Example 1.The resulting utensil has microwave heating and thermal cyclingproperties, abrasion resistance and detergent resistance similar to theutensil of Example 1. However, the utensil of this example resistsformation of arcs and localied hot spots during exposure to microaveradiation beter than the utensil of Example 1.

What is claimed is:
 1. A microwave heating utensil comprising asubstantially microwave-transparent body and a particulate susceptormaterial fixed to said body, said susceptor material comprising zinc andone or more zinc antioxidant metals.
 2. A microwave heating utensilcomprising a substantially microwave-transparent body and a particulatesusceptor material fixed to said body, said susceptor materialcomprising zinc and manganeses.
 3. A utensil as claimed in claim 2wherein said susceptor material is disposed in a susceptor layer on asurface of said body.
 4. A utensil as claimed in claim 3, furthercomprising an electrically non-conductive moderator in said susceptorlayer.
 5. A microwave heating utensil as claimed in claim 4 furthercomprising a continuous cover layer of a substantiallymicrowave-transparent cover material closely overlying said susceptorlayer, said cover layer being fused to said body.
 6. A utensil asclaimed in claim 5 wherein said body is formed from a glass or ceramicmaterial and said cover material consists essentially of a glass orceramic material.
 7. A utensil as claimed in claim 6 wherein said bodyis formed from a glass-ceramic material and said cover layer consistsessentially of glass.
 8. A utensil as claimed in claim 4 wherein theratio of zinc to manganese in said susceptor material is between about80:8 and about 80:18 by weight.
 9. A utensil as claimed in claim 8wherein said ratio of zinc to manganese is about 80:13.
 10. A utensil asclaimed in claim 8 wherein the metal mass per unit area of saidsusceptor layer is about 4 mg/cm² to about 10 mg/cm².
 11. A method ofmaking a microwave heating utensil comprising the steps of:(a) providinga body; (b) applying a susceptor layer of a particulatesusceptor-forming material including zinc and one or more zincantioxidant metals over a surface of said body; (c) applying a coverlayer of a particulate fusible cover material over said susceptor layer;(d) heating said layers so as to fuse said cover material to form acontinuous cover layer and ufse said cover layer to said body, therebyforming a unitary utensil; and (e) cooling said utensil.
 12. A microwaveheating utensil made by a method as claimed in claim
 11. 13. A method ofmaking a microwave heating utensil comprising the steps of:(a) providinga body; (b) applying a susceptor layer of a particulatesusceptor-forming material including zinc and manganese over a surfaceof said body; (c) applying a cover layer of a particulate fusible covermaterial over said susceptor layer; (d) heating said layers so as tofuse said cover material to form a continuous cover layer and fuse saidcover layer to said body, thereby forming a unitary untensil; and (e)cooling said utensil.
 14. A method as claimed in claim 13 wherein bothsaid susceptor layer and said cover layer are provided as performedlayers on a decal substrate and are applied to said body by transferringsaid performed layers from said decal substrate to said body.
 15. Amethod as claimed in claim 13 wherein said body is formed from a glassor ceramic material and said cover material consists essentially of aglass or ceramic material.
 16. A method as claimed in claim 13 whereinsaid susceptor-forming material includes zinc and manganese in thereduced state, and said heating step is performing in an oxidizingatmosphere.
 17. A method as claimed in claim 16 wherein said susceptorlayer includes a particulate fusible moderator, in addition to saidsusceptor-forming material, said moderator being fused during saidheating step.
 18. A method as claimed in claim 17 wherein saidsusceptor-forming material consists essentially of zinc and manganese.19. A method as claimed in claim 18 wherein the ratio of zinc tomanganese by weight in said susceptor-forming material is about 80:8 toabout 80:18.
 20. A method as claimed in claim 19 wherein said moderatorconsists essentially of glass and the ratio of said susceptor-formingmaterial to said moderator by weight in said susceptor layer prior tosaid heating step is between about 88:12 and about 98:2.
 21. A method asclaimed in claim 20 wherein said ratio of zinc to manganese is saidsusceptor-forming material is about 80:13.
 22. A method as claimed inclaim 21 wherein said ratio of susceptor-forming material to moderatoris about 93:7.
 23. A method as claimed in claim 22 wherein the meanparticle sizes of said susceptor-forming material and said moderator areabout 20 microns or less.
 24. A microwave heating utensil made by amethod as claimed in claim
 23. 25. A decal for making a microwaveheating utensil comprising a substrate, and a susceptor layer comprisinga particulate susceptor-forming material including zinc and one or morezinc antioxidant metals secured to said substrate.
 26. A decal formaking a microwave heating utensil comprising a substrate, and asusceptor layer comprising a particulate susceptor-forming materialincluding zinc and manganese secured to said substrate.
 27. A decal asclaimed in claim 35 further comprising a cover layer including aparticulate fusible cover material consisting essentially of a glass orceramic material disposed adjacent said susceptor layer.
 28. A decal asclaimed in claim 27 wherein said susceptor layer includes a particulate,fusible electrically non-conductive moderator in addition to saidsusceptor-forming material.
 29. A decal as claimed in claim 28 whereinthe ratio of zinc to manganese by weight in said susceptorformingmaterial is about 80:8 to about 80:18.
 30. A decal as claimed in claim29 wherein said moderator consists essentially of glass and the ratio ofsaid susceptor-forming material to said moderator by weight in saidsusceptor layer prior to said heating step is between about 88:12 andabout 98:2.
 31. A decal as claimed in claim 30 wherein said ratio ofzinc to manganese in said susceptor-forming material is about 80:13. 32.A decal as claimed in claim 31 wherein said ratio of susceptor-formingmaterial to moderator is about 93:7.
 33. A decal as claimed in claim 32wherein said susceptor layer includes between about 4 mg/cm² and about10 mg/cm² of said susceptor-forming material.